Electron transfer and chloride ligand dissociation in complexes [(C(5)Me(5))ClM(bpy)](+)/[(C(5)Me(5))M(bpy)](n) (M=Co, Rh, Ir; n=2+,+,0,-): A combined electrochemical and spectroscopic investigation

In contrast to the rapid and chemically reversible two-electron ECE' reductive elimination reaction [(C(5)Me(5))ClM(bpy)](+) + 2e(-) --> (C(5)Me(5))M(bpy) + Cl-, M = Rh or Ir, the analogous cobalt system exhibits two separate one-electron steps (EC + E' process) with a persistent, EPR-spectroscopically characterized cobalt(II) intermediate [(C(5)Me(5))Co(bpy)](+). Within the series of coordinatively unsaturated homologous species (C,Me,)M(bpy), the cobalt derivative exhibits the smallest and the iridium homologue the largest metal(I)-to-bpy electron transfer in the ground state, as evident from electrochemical potentials and long-wavelength absorption data. A comparison within that homologous series indicates why the rhodium system, with its intermediate position, is most suitable for hydride transfer catalysis.